Humans have unintentionally changed the world and turned up the temperature. A climate compromise might turn down the heat.

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Geo-engineering, the deliberate alteration of the planet to undo its inadvertent alteration by humans over the past 200 years, is back on the scientific agenda, with a climate compromise suggested as a possible solution.

One group wants to turn down the global thermostat and reverse the global warming trend set in train by greenhouse gases released by fossil fuel combustion, by thinning the almost invisible cirrus clouds that trap radiation and keep the planet warm.

Another group proposes to inject sulphur particles into the stratosphere, and keep on doing so for 160 years, to block enough sunlight and lower the planetary temperature.

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The same amount of rain fell around the globe in our models, but it fell in different places, which could create a big mismatch between what our economic infrastructure expects and what it will get.

Ken Caldeira, Carnegie Institution for Science

And a third group wants to see a cocktail of both approaches: thin the high cirrus clouds that stop heat from escaping, and at the same pump particles into the stratosphere to scatter the incoming sunlight and limit the disadvantages of each approach by mixing them.

The verb “wants” in all three studies is neither fair nor appropriate: all three groups concede that the healthy answer is for humans to fulfil the pledge made in 2015, and start to reduce fossil fuel emissions so drastically that global average temperatures stay well below the 2°C maximum rise agreed by 197 nations at the Paris climate conference.

Change inevitable

But while most nations have yet to deliver on the plans they have made, and some nations have yet to even devise a plan, and one nation – the US – has announced its withdrawal from the agreement, scientists have been looking for ways to reverse the potentially catastrophic warming and climate change that is now inevitable if the world continues with its “business as usual scenario.”

And so, tentatively, and with unpromising conclusions, researchers have looked at ways to alter the planet to protect it from rising temperatures.

They have repeatedly investigated ways of reducing the incoming sunlight, usually by pumping sulphate aerosols into the atmosphere, and they have even investigated the possibility of making the ocean more thirsty for carbon dioxide, the most problematic greenhouse gas, by pumping iron into the sea to nourish the photosynthesising algae.

But by the end of the century, as sea levels rise by a metre and global average temperatures by 4°C or more, even a bad answer could be the only one on offer. So Ulrike Lohmann and Blaz Gasparini, two scientists from the Swiss Technical Institute known as ETH Zurich, write in the journal Science that the answer might lie in the clouds.

Cirrus clouds in particular don’t reflect much sunlight back into space, but because of the altitude and the temperature they do emit less long-wave radiation: they behave, in effect, rather like greenhouse gases.

So if cirrus clouds were carefully created by artificial means at lower altitudes, then perhaps they would trap less heat.

Such an experiment, the scientists concede, could go badly wrong, would not solve problems linked to rainfall patterns and might even make the world warmer. For the moment, they say: “cirrus cloud thinning should be viewed as a thought experiment.”

And in the same journal two scientists from the Max Planck Institute for Meteorology in Hamburg, Germany, and the US National Centre for Atmospheric Research in Boulder, Colorado, look once again at the stratosphere solution: what sulphur particles could do to cool the planet.

To limit the temperature increase to 2°C above pre-industrial levels, humans would have to pump sulphur into the stratosphere at a cost of $20bn a year for 160 years, to darken the skies and reduce incoming sunlight.

Side effects

Such a step, which could slow the water cycle and suppress the Asian monsoons, would not reduce the acidification of the oceans,and could trigger other unwelcome side effects that could lead to global conflicts.So, the scientists say, any such plan would need international agreement and supervision.

And, in the journal Geophysical Research Letters, Chinese, Indian and US researchers arrive at the compromise solution. The dark skies approach could reduce rain too much, thinner cirrus could reduce rain too little.

But computer models suggest that if both methods were deployed carefully and in concert, geo-engineers could cool the world but keep the rainfall steady overall.

“The same amount of rain fell around the globe in our models, but it fell in different places, which could create a big mismatch between what our economic infrastructure expects and what it will get,” said Ken Caldeira of the Carnegie Institution for Science in the US, and one of the authors.

“More complicated geo-engineering solutions would likely do a bit better, but the best solution is simply to stop adding greenhouse gases to the atmosphere.”

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